An active-matrix-addressed liquid crystal display device generally includes a substrate on which thin-film transistors (which will also be referred to herein as “TFTs”) are provided as switching elements for respective pixels (such a substrate will be referred to herein as a “TFT substrate”), a counter substrate on which color filters and other members are arranged, and a liquid crystal layer which is interposed between the TFT substrate and the counter substrate. The TFT substrate further includes storage capacitors along with the TFTs. Each of those storage capacitors is a capacitance which is arranged, in order to maintain the voltage applied to the liquid crystal layer of a pixel (which is called electrically a “liquid crystal capacitor”), electrically in parallel with the liquid crystal capacitor. In this description, a TFT substrate and a display device including a TFT substrate will be sometimes referred to herein as “semiconductor devices”.
Patent Document No. 1 discloses a liquid crystal display device including a storage capacitor with a transparent storage capacitor electrode (which will be sometimes referred to herein as a “transparent storage capacitor”). A liquid crystal display device with such a storage capacitor can use incoming light highly efficiently and can obtain sufficient storage capacitance without causing a decrease in the aperture ratio of the pixel. Thus, it is said that such a liquid crystal display device is applicable to making a liquid crystal display device which has had significantly increased definition.
Recently, people have proposed that an oxide semiconductor be used as a material for the active layer of a TFT instead of a silicon semiconductor. Such a TFT will be referred to herein as an “oxide semiconductor TFT”. Since an oxide semiconductor has higher mobility than amorphous silicon, the oxide semiconductor TFT can operate at higher speeds than an amorphous silicon TFT. For example, Patent Document No. 2 discloses an active-matrix-addressed liquid crystal display device which uses an oxide semiconductor TFT as a switching element (see Patent Document No. 2, for example). Also, the oxide semiconductor TFT disclosed in Patent Document No. 2 includes an etch stop layer over an oxide semiconductor layer in order to protect the channel region of the oxide semiconductor layer.